A prior art smoke sensing apparatus of the above type has such a construction that light emitted from a light emitting element and scattered by smoke is converted by a light receiving element into an electrical signal which is applied, after being amplified by an amplifier, to a comparator to be compared with a predetermined level. The comparator generates a control signal actuating an air conditioner when the level of the comparator input signal becomes higher than the predetermined level. However, the prior art smoke sensing apparatus has had the following problems: Firstly, the sensitivity of the smoke sensing apparatus tends to change depending on the temperature due to temperature-dependent variations of the characteristics of the elements including the light emitting and receiving elements and amplifier. Secondly, the level of the comparator input signal tends to vary due to degradations or secular variations of the characteristics of the light emitting and receiving elements due to a change in the quantity of reflected light not attributable to smoke, resulting in a reduced sensitivity or mal-operation of the apparatus due to the signal level variation.